This invention relates generally to high voltage circuit breakers, and more specifically to methods and systems for analyzing circuit breaker contacts.
At least some known circuit breakers, use a pre-insertion resistor to facilitate protecting circuits during closing operations of the circuit breaker. Specifically, as the circuit breaker is closed, the pre-insertion resistor is connected in parallel with a gap defined between the open circuit breaker contacts. More specifically, when the pre-insertion resistor is placed in parallel with the gap, the circuit voltage measured to ground (generally line-to-ground voltage) is dropped across the resistor. Accordingly, the current flowing through the resistor is determined by V/Z, wherein V represents the line-to-ground voltage of the circuit and Z represents the vector sum of the resistance of the pre-insertion resistor and the surge impedance of inductive and capacitive elements, such as capacitor banks, reactors, and bus work coupled to and within the circuit. The current determined by this calculation is often referred to as the inrush current and may momentarily achieve a substantially high level when the circuit breaker is used in conjunction with a capacitor bank.
During operation, inrush currents with relatively large magnitudes may cause damage to the circuit. For example, without pre-insertion resistors, the inrush current may reach values of about 10 to 30 thousand amperes. However, with a pre-insertion resistor installed, the initial inrush current may reach values of only about 2 to 4 thousand amperes. Following the initial inrush current, the current through the pre-insertion resistor may be limited by the steady state impedance of capacitor banks, and other circuit components, such as, but not limited to loads, reactors, and bus work coupled to the circuit. Consequently, following the initial inrush current, the current flow through the pre-insertion resistor is generally within the range of 100 to 400 amperes. Additionally, after the initial inrush current has subsided, and the current through the pre-insertion resistor has been reduced dropped to a substantially lower level of 100–400 amperes, contacts of the circuit breaker quickly re-engage. If the circuit breaker is switching capacitor banks, the banks discharge directly through the contacts, such that the current is limited by the surge impedance of the banks and the bus work.
The engagement of the main contacts shunts the majority of the circuit breaker current through the main contacts so that the pre-insertion resistor does not carry a substantial part of the continuous or normal current through the circuit breaker. Therefore, the timing of the closure of circuit breaker contacts and the resistive integrity of the pre-insertion resistor are factors that may facilitate enhanced operation of a high voltage circuit breaker. Accordingly, verifying such parameters by periodic testing may facilitate proper circuit breaker operation. However, such testing is typically performed in place, for example, in a switchyard or substation where the circuit breaker is normally located, and may subject testing equipment to power line frequency interference from voltages induced into test equipment components and/or cabling from power lines located proximate the circuit breaker and testing equipment. Additionally, when timing a circuit breaker in a substation with high voltage lines surrounding the circuit breaker being tested, a power line frequency current flow may be undesirably induced into the measurement circuits due to capacitive coupling between the test object and adjacent high voltage lines. The interference current may be of substantially the same frequency as the desired measurement result, therefore adversely affecting the measurement result.